Radar can be considered as a device for measuring and communication. In a traditional pulsed radar, the radar sends out a pulse and measures the time for the echo from the reflecting object to reach the radar receiver. Echoes from different objects can have different appearances, for example, moving objects give a Doppler shift of frequency, and by analyzing the echoes, certain information about the reflecting object can be obtained. If the echo analysis can be said to constitute the measuring function of the radar, then the transmission of the pulse itself to the reflecting object and the transmission of the echo response back to the radar can be said to constitute the communication part of the radar.
In a way corresponding to radio communication, the radar's communication function is sensitive to interference and limited in range by wave propagation characteristics such as atmospheric attenuation, curvature of the earth and so forth.
The communication characteristics of the radar are particularly significant in military applications. Countermeasures which can prevent radar functioning are constantly being developed and the communication characteristics of the radar are critical with regard to how vulnerable the radar is to different countermeasure systems.
The radar can be disturbed actively or passively. In active interference, power is generated within the frequency range of the radar and is transmitted towards the radar which thereby experiences false echoes, which are often of such an amplitude and persistence that the receiver function is completely blanked out within certain parts of the radar coverage area. Passive interference involves creating decoys, for example by creating "clouds" of reflecting strips. The echoes from the strips can be so great that echoes from aeroplanes and boats do not stand out within the strip area.
In both passive and active interference, the party producing the interference thereby exposes its presence and its need for protective activity. It is frequently an important part of strategy to expose as little as possible of one's activities to the enemy. This means using neither radar nor countermeasures against radar until this is tactically justified.
To spot radar, signal interception devices are used. These are found in many versions, from simple warning receivers to highly advanced interception equipment with advanced analyzing functions.
Most of the signal interception equipment is designed for detecting radar pulses by a wide-band method which provides poor receiver sensitivity compared with a radar receiver. However, the radar loses more signal power due to the fact that the signal must go out and back to reach the radar receiver but only go one way to reach the signal interception equipment. This relation results in that the signal interception equipment can detect the radar at a longer range than the radar can see the actual target.
What has been said applies to traditional pulsed radars where the peak pulse power is much greater than the average power of the radar. The signal interception equipment is designed to sense the peak power, while the radar receiver senses the average power.
It is already known to use continuous wave (CW) radar. This has special characteristics, for example as warning radar for fast moving targets. If the radar transmits on one frequency, there must be a Doppler shift in the frequency of the echo signal for the received signal to be detected. This radar therefore does not provide any possibility for determining distance to the target. Other ideas for the CW radar have also been found. Most of the attempts with CW radar, however, have not led to any success, which is why radar signal interception equipment is frequently not equipped to spot such a radar. Moreover, for a CW radar it holds true that the peak power is equal to average power, which has the result that the signal interception equipment with its poor sensitivity can have a shorter range than the corresponding own range of the CW radar. That is to say that the radar can operate without being spotted. The radar is "quiet".
This is a very interesting characteristic of the radar since it must be known that there is a radar, the direction in which it is located and on what frequencies it transmits in order to be able to define a disturbance of the radar.
It is already known to be able to obtain range information from CW radar, for example by coupling together two or more radars to obtain a calculated point for the echo by interception technique. Another way to obtain range information by means of one radar is to use so-called frequency modulated continuous wave (FMCW) radar where the frequency of the transmitted carrier wave is changed (increased) continuously with time. An example of this type of radar is the PILOT radar developed by Philips. Like conventional CW radar, the FMCW radar is difficult to spot and is "silent". However, the risk that this type of radar can also be disturbed is increased as a result of more sophisticated countermeasures. Besides, the frequency modulation of the FMCW radar can provide an error in range determination upon detection of moving targets.
There is another type of radar on the market which should be mentioned in this connection, namely the so-called travelling wave tube (TWT) radar. Unlike a traditional pulsed radar, where the radar transmitter comprises a magnetron tube which transmits short pulses of the order of magnitude of 0.05-5 .mu.s with high power, a travelling wave tube is utilized in TWT radar. Such a tube cannot provide peak powers of the same magnitude as a magnetron, which is why the pulse durations need to be increased in order to obtain corresponding avarage powers. The advantage with TWT radar is that it can provide good MTI characteristics. The disadvantage is the poor range resolution compared with conventional pulsed radar.
To increase the range resolution in TWT radar, it is already known to vary the frequency or phase in the transmitted pulse. This can be done by analog or digital means and provides a certain pulse compression which increases the range resolution of the radar.
The TWT radar is also a pulsed radar which can be easily spotted by signal interception equipment.